1. Field of the Invention
The present invention relates to a frequency shifter and an optical displacement measurement apparatus using the same and, more particularly, to a frequency shifter and an optical displacement measurement apparatus using the same, which are suitably used to measure the displacement information of a moving object or fluid (to be referred to as "a moving object" hereinafter) in a non-contact manner by detecting a shift in the frequency of each scattered light beam having undergone a Doppler shift in accordance with the moving velocity of the moving object.
2. Description of the Related Art
Conventionally, as an apparatus for precisely measuring the displacement information of a moving object in a non-contact manner, a laser Doppler velocimeter or laser encoder (optical displacement measurement apparatus) has been used. The laser Doppler velocimeter measures the moving velocity of a moving object by utilizing an effect (Doppler effect) that when a laser beam is irradiated onto a moving object, the frequency of the laser beam scattered by the moving object shifts in proportion to the moving velocity.
The present applicant proposed such laser Doppler velocimeters in, e.g., Japanese Laid-Open Patent Application Nos. 2-262064 and 4-230885.
The following method has been described by Foord et al., (Appl. Phys., Vol. 7, 1974, pp. 136-139). In a laser Doppler velocimeter, a frequency shifter using flat plates consisting of electro-optic crystals (to be referred to as "electro-optic elements" hereinafter) is arranged on the optical paths of two light beams, and the two light beams are incident on a moving object with a predetermined frequency difference being set between the light beams by means of the frequency shifter. According to this method, even if the moving direction and velocity of a moving object are close to zero, the moving velocity can be measured with high precision.
FIG. 1A is a schematic view showing the main part of a laser Doppler velocimeter using the above detection principle. Referring to FIG. 1A, a frequency shifter 100 is constituted by two electro-optic elements 10a and 10b, their driving circuits, and the like.
A parallel light beam 3 having a wavelength .lambda., which is emitted from a light source, is split into two light beams 5a and 5b by a beam splitter 4. The respective light beams are incident on the electro-optic elements 10a and 10b constituting the frequency shifter 100.
In this case, the light beams 5a and 5b undergo frequency shifts based on the sawtooth waveform voltage driving (serrodyne driving) of the electro-optic elements 10a and 10b. With this operation, a frequency difference is imparted to the two light beams 5a and 5b, and the two light beams are deflected by a lens 6 to be incident on a moving object, which is moving in the direction indicated by the arrow at a velocity V.sub.0, in such a manner that the two light beams cross each other at an incident angle .theta.. When light scattered by the moving object, of the incident light beams, is guided to a photodetector, a Doppler signal can be obtained from the photodetector.
In this case, the frequencies of the scattered light beams of the two light beams are subjected to Doppler shifts in proportion to the moving velocity V.sub.0 to cause interference on a detection surface, thereby causing a change in density pattern. The frequency of this change in density pattern, i.e., a Doppler frequency F, is given by EQU F=2.multidot.V.sub.0 .multidot.sin(.theta.)/.lambda.+f.sub.R ( 1)
where f.sub.R is the frequency difference between the two light beams. With this method, even if the velocity V.sub.0 of the moving object is low or close to zero, the moving velocity and direction of the moving object can be simultaneously measured by setting a proper value as the frequency difference f.sub.R.
In general, when highly coherent light such as a laser beam is irradiated onto an object, light scattered by a fine irregularity on a surface of the object is randomly phase-modulated to form a dot pattern, i.e., a so-called speckle pattern, on the observation surface. In the laser Doppler velocimeter, when an object moves, a change in density pattern due to the Doppler shift on the detection surface of the photodetector is modulated by an irregular change in density pattern due to the flow of the speckle pattern, and the output signal from the photodetector is also modulated by a change in transmittance (or reflectance) of an object to be measured.
In the laser Doppler velocimeter, the frequency of change in density pattern due to the flow of a speckle pattern and the frequency of change in transmittance (or reflectance) of a moving object are generally lower than a Doppler frequency based on displacement information. For this reason, the output from the photodetector is supplied to a high-pass filter to remove the low-frequency components to extract only a Doppler signal.
If, however, the velocity of the moving object is low and the Doppler frequency is low, the frequency difference between the Doppler frequency and the low-frequency variation components becomes small, and a high-pass filter cannot be used. This makes it difficult to measure the displacement information of the moving object with high precision. In addition, the velocity direction cannot be detected in principle.
In contrast to this, according to the above method (frequency shifter) disclosed by Foord et al., a predetermined frequency difference is set between two light beams before the light beams are irradiated onto a moving object to allow measurement of the stationary state and velocity direction of the moving object.
Frequency shifters having various arrangements are available. If, for example, a frequency shift using electro-optic crystals are applied to the above method, an ordinary index n.sub.o and an extraordinary index n.sub.e of each electro-optic crystal greatly change with a change in temperature. As shown in FIG. 1B, in particular, the refractive index of the electro-optic crystal is very sensitive to temperature. For this reason, a frequency shift becomes unstable with a slight temperature difference or temperature gradient of the electro-optic element. In addition, if a temperature gradient occurs at a light-transmitting portion, the light beam wave surface is distorted. As a result, a strict interference system is difficult to form.
Owing to the above problems, difficulties have been posed in realizing practical applications of a frequency shifter, as a bulk type device, to which electro-optic crystals are applied. Furthermore, in order to cause a slight frequency shift, a voltage applied to each electro-optic element must be decreased. If, however, the voltage applied to the electro-optic element 10a is decreased, a structure near the electro-optic element is complicated.